Alveolar pressure

Distinguish among atmospheric pressure, alveolar pressure, intrapleural pressure, and transpulmonary pressure... 

Alveolar pressure (Palv) is the pressure within the alveoli. In between breaths, it is equal to 0 cmH20. Because no pressure gradient exists between the atmosphere and the alveoli, there is no airflow. However, in order for air to flow into the lungs, alveolar pressure must fall below atmospheric pressure. In other words, alveolar pressure becomes slightly negative. According to Boyle s law, at a constant temperature, the volume of a gas and its pressure are inversely related ... 

Airflow through the airways is proportional to the gradient between atmospheric pressure and alveolar pressure (AP) and inversely proportional to the airway resistance (R). 

Crack, the heat stable form of cocaine, when smoked and followed by deep inhalation plus a Valsalva maneuver to increase uptake, and cough triggered by the sniffed substance can cause pulmonary barotrauma. The increased intra-alveolar pressure can cause alveolar rupture, with consequent air dissection through the peribronchial connective tissue in the mediastinum, pleural space, pericardium, peritoneum, or subcutaneous soft tissues. 

Her respiratory problems could have been due to alveolar rupture, caused by an increase in intra-alveolar pressure, due to the exertion while she had been dancing strenuously. Alternatively, it could have been secondary to her use of positive ventilatory pressure after taking the drugs this is done by a partner, either by direct mouth-to-mouth contact or through a cardboard cylinder, to enhance the user s experience of the stimulant s effects. 

Perfusion Perfusion occurs when blood from the pulmonary circulation is sufficient at the alveolarcapillary bed to conduct diffusion. For perfusion to occur, the alveolar pressure must be matched by adequate ventilation. Mucosal edema, secretions, and bronchospasm increase resistance to the airflow, resulting in decreased ventilation and decreased diffusion. 

Normal expiration is a passive process, and when the inspiratory muscles end their contraction, the elastic recoil of the lung pulls the lung back to its original size and shape. This process makes the alveolar pressure positive relative to the pressure at the mouth, and air flows out of the lung. During inspiration, the respiratory muscles must overcome the elastic properties of the lung (elastic recoil) and the resistance to airflow by the airways. During expiration, the flow of air is determined primarily by the elastic recoil and airway resistance. 

To measure compliance, the subject is intubated with an esophageal balloon and then told to breathe in or out of a spirometer in 500-ml increments. During breath holding, with the epiglottis open, the alveolar pressure is the same as the pressure at the mouth. Values of volume change and pleural pressure changes measured simultaneously produce a pressure-volume curve similar to that shown in Fig. 13. 

There is no true pulmonary analogue to the systemic arterioles, since the pulmonary circulation occurs under relatively low pressure [West, 1977]. Pulmonary blood vessels, especially capiQaries and venules, are very thin walled and flexible. Unlike systemic capillaries, pulmonary capillaries increase in diameter, and pulmonary capillaries within alveolar walls separate adjacent alveoli with increases in blood pressure or decreases in alveolar pressure. Flow, therefore, is significantly influenced by elastic deformation. Although pulmonary circulation is largely unaffected by neural and chemical control, it does respond promptly to hypoxia. 

At the same time, alveolar pressure can be correlated to plethysmographic pressure. Therefore, when the shutter is again opened and flow rate is measured, airway resistance can be obtained as the ratio of alveolar pressure (obtainable from plethysmographic pressure) to flow rate [Carr and Brown, 1993]. Airway resistance is usually measured during panting, at a nominal lung volume of FRC and flow rate of =bl 1/sec. 

Purmutt S., Bromberger-Barnea B. and Bane H.N. 1962. Alveolar pressure, pulmonary venous pressure, and the vascular waterfall. Med. Thorac. 19 239. 

If the wheel spins rapidly enough so that each partial obstruction is short, the alveolar pressure is assumed to remain constant. Solving these equations yields... 

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